title: Evidence that the platelet plasma membrane is impermeable to calcium and magnesium complexes of A23187. A23187-induced secretion is inhibited by MG2+ and Ca2+, and requires aggregation and active cyclooxygenase.
abstract: A23187-treated platelets secrete dense granule constituents during centrifugation, an artifact that is presented by prior formalin fixation (Holmsen, H., and Setkowsky-Dangelmaier, C. A. (1977) Biochim. Biophys. Acta 497, 46-61). With this improved assay, A23187 induced no secretion in nonaggregating platelets and maximal secretion in aggregating platelets within 3 min. Further incubation gave a slow, submaximal secretion in nonaggregating cells. Acetylsalicylate abolished secretion in both systems. EDTA, but not ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, strongly enhanced secretion in nonaggregating platelets suspended in Mg2+-containing, Ca2+-free Tyrode's solution. Without added Mg2+, A23187 gave maximal secretion in nonaggregating platelets which was abolished by added Mg2+. Preincubation of A23187 and MgCl2 gave inhibition patterns which clearly suggested that formation of Mg.A23187 species was the cause of inhibition. Ca2+, but not Sr2+, inhibited A23187-induced secretion in the same manner as Mg2+. These findings suggest that the platelet plasma membrane has no or very little permeability for Ca2+ . and Mg2+ . A23187 species. In the physiological suspending medium, Ca2+-free Tyrode's solution, A23187-induced platelet secretion is markedly enhanced by close cell contact (aggregation) and has an absolute requirement for production of prostaglandins and/or thromboxanes. Therefore, the widely held view that secretion is directly triggered by A23187-induced increase in the cytoplasmic Ca2+ concentration is not applicable to platelets.
mesh_terms: D000900:Anti-Bacterial Agents / Q000097:blood*; D001241:Aspirin / Q000494:pharmacology; D001792:Blood Platelets / Q000378:metabolism*; D000001:Calcimycin / Q000097:blood* / Q000494:pharmacology; D002118:Calcium / Q000097:blood* / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D002463:Cell Membrane Permeability; D006801:Humans; D007700:Kinetics; D008274:Magnesium / Q000097:blood* / Q000494:pharmacology; D010974:Platelet Aggregation / Q000187:drug effects*; D011451:Prostaglandin-Endoperoxide Synthases / Q000378:metabolism*; D012701:Serotonin / Q000097:blood; D013324:Strontium / Q000494:pharmacology
chemical_list: D000900:Anti-Bacterial Agents; D012701:Serotonin; D000001:Calcimycin; D011451:Prostaglandin-Endoperoxide Synthases; D008274:Magnesium; D001241:Aspirin; D002118:Calcium; D013324:Strontium